Cleaning Someone Else’s Home: Helpful or Creepy?

We were babysitting last night and found ourselves in the midst of an unusual dilemma which caused us to have flashbacks to a certain episode of The Big Bang Theory. After putting the kids to bed we started cleaning up the mess we'd made and were tempted to just keep cleaning! Is cleaning someone else's house without their asking a kind gesture or just plain creepy?

After lots of giggling and quoting "if you have time to lean, you have time to clean" and some hard thinking we decided that in this situation our gesture might not have been taken as how we had intended it. We understand completely that with children, fulltime work plus study things can spiral out of control at a rapid pace [we struggle without children], but is there a line in the sand dust you shouldn't cross? Is washing a sink full of dishes acceptable behavior or is it on par with folding someone else's underwear?

Have you cleaned someone's house without their invitation or have you come home to find your house had been cleaned? How did you react?